Microelectronic programmable device and methods of forming and programming the same

ABSTRACT

A microelectronic programmable structure and methods of forming and programming the structure are disclosed. The programmable structure generally include an ion conductor and a plurality of electrodes. Electrical properties of the structure may be altered by applying a bias across the electrodes, and thus information may be stored using the structure.

FIELD OF THE INVENTION

[0001] The present invention generally relates to microelectronicdevices. More particularly, the invention relates to programmablemicroelectronic structures suitable for use in integrated circuits.

BACKGROUND OF THE INVENTION

[0002] Memory devices are often used in electronic systems and computersto store information in the form of binary data. These memory devicesmay be characterized into various types, each type having associatedwith it various advantages and disadvantages.

[0003] For example, random access memory (“RAM”) which may be found inpersonal computers is typically volatile semiconductor memory; in otherwords, the stored data is lost if the power source is disconnected orremoved. Dynamic RAM (“DRAM”) is particularly volatile in that it mustbe “refreshed”(i.e., recharged) every few microseconds in order tomaintain the stored data. Static RAM (“SRAM”) will hold the data afterone writing so long as the power source is maintained; once the powersource is disconnected, however, the data is lost. Thus, in thesevolatile memory configurations, information is only retained so long asthe power to the system is not turned off In general, these RAM devicescan take up significant chip area and therefore may be expensive tomanufacture and consume relatively large amounts of energy for datastorage. Accordingly, improved memory devices suitable for use inpersonal computers and the like are desirable.

[0004] Other storage devices such as magnetic storage devices (e.g.,floppy disks, hard disks and magnetic tape) as well as other systems,such as optical disks, CD-RW and DVD-RW are non-volatile, have extremelyhigh capacity, and can be rewritten many times. Unfortunately, thesememory devices are physically large, are shock/vibration-sensitive,require expensive mechanical drives, and may consume relatively largeamounts of power. These negative aspects make such memory devicesnon-ideal for low power portable applications such as lap-top andpalm-top computers, personal digital assistants (“PDAs”), and the like.

[0005] Due, at least in part, to a rapidly growing numbers of compact,low-power portable computer systems in which stored information changesregularly, low energy read/write semiconductor memories have becomeincreasingly desirable and widespread. Furthermore, because theseportable systems often require data storage when the power is turnedoff, non-volatile storage device are desired for use in such systems.

[0006] One type of programmable semiconductor non-volatile memory devicesuitable for use in such systems is a programmable read-only memory(“PROM”) device. One type of PROM, a write-once read-many (“WORM”)device, uses an array of fusible links. Once programmed, the WORM devicecannot be reprogrammed.

[0007] Other forms of PROM devices include erasable PROM (“EPROM”) andelectrically erasable PROM (EEPROM) devices, which are alterable afteran initial programming. EPROM devices generally require an erase stepinvolving exposure to ultra violet light prior to programming thedevice. Thus, such devices are generally not well suited for use inportable electronic devices. EEPROM devices are generally easier toprogram, but suffer from other deficiencies. In particular, EEPROMdevices are relatively complex, are relatively difficult to manufacture,and are relatively large. Furthermore, a circuit including EEPROMdevices must withstand the high voltages necessary to program thedevice. Consequently, EEPROM cost per bit of memory capacity isextremely high compared with other means of data storage. Anotherdisadvantage of EEPROM devices is that, although they can retain datawithout having the power source connected, they require relatively largeamounts of power to program. This power drain can be considerable in acompact portable system powered by a battery.

[0008] In view of the various problems associated with conventional datastorage devices described above, a relatively non-volatile, programmabledevice which is relatively simple and inexpensive to produce is desired.Furthermore, this memory technology should meet the requirements of thenew generation of portable computer devices by operating at a relativelylow voltage while providing high storage density and a low manufacturingcost.

SUMMARY OF THE INVENTION

[0009] The present invention provides improved microelectronic devicesfor use in integrated circuits. More particularly, the inventionprovides relatively non-volatile, programmable devices suitable formemory and other integrated circuits.

[0010] The ways in which the present invention addresses variousdrawbacks of now-known programmable devices are discussed in greaterdetail below. However, in general, the present invention provides aprogrammable device that is relatively easy and inexpensive tomanufacture, and which is relatively easy to program.

[0011] In accordance with one exemplary embodiment of the presentinvention, a programmable structure includes an ion conductor and atleast two electrodes. The structure is configured such that when a biasis applied across two electrodes, one or more electrical properties ofthe structure change. In accordance with one aspect of this embodiment,a resistance across the structure changes when a bias is applied acrossthe electrodes. In accordance with other aspects of this embodiment, acapacitance or other electrical property of the structure changes uponapplication of a bias across the electrodes. One or more of theseelectrical changes may suitably be detected. Thus, stored informationmay be retrieved from a circuit including the structure.

[0012] In accordance with another exemplary embodiment of the invention,a programmable structure includes an ion conductor, at least twoelectrodes, and a barrier interposed between at least a portion of oneof the electrodes and the ion conductor. In accordance with one aspectof this embodiment the barrier material includes a material configuredto reduce diffusion of ions between the ion conductor and at least oneelectrode. The diffusion barrier may also serve to prevent undesiredelectrodeposit growth within a portion of the structure. In accordancewith another aspect, the barrier material includes an insulatingmaterial. Inclusion of an insulating material increases the voltagerequired to reduce the resistance of the device. In accordance with yetanother aspect of this embodiment, the barrier includes material thatconducts ions, but which is relatively resistant to the conduction ofelectrons. Use of such material may reduce undesired plating at anelectrode and increase the thermal stability of the device.

[0013] In accordance with another exemplary embodiment of the invention,a programmable microelectronic structure is formed on a surface of asubstrate by forming a first electrode on the substrate, depositing alayer of ion conductor material over the first electrode, and depositingconductive material onto the ion conductor material. In accordance withone aspect of this embodiment, a solid solution including the ionconductor and excess conductive material is formed by dissolving (e.g.,via thermal and/or photodissolution) a portion of the conductivematerial in the ion conductor. In accordance with a further aspect, onlya portion of the conductive material is dissolved, such that a portionof the conductive material remains on a surface of the ion conductor toform an electrode on a surface of the ion conductor material.

[0014] In accordance with another embodiment of the present invention,at least a portion of a programmable structure is formed within athrough-hole or via in an insulating material. In accordance with oneaspect of this embodiment, a first electrode feature is formed on asurface of a substrate, insulating material is deposited onto a surfaceof the electrode feature, a via is formed within the insulatingmaterial, and a portion of the programmable structure is formed withinthe via. After the via is formed within the insulating material, aportion of the structure within the via is formed by depositing an ionconductive material onto the conductive material, depositing a secondelectrode material onto the ion conductive material, and, if desired,removing any excess electrode, ion conductor, and/or insulatingmaterial. In accordance with another aspect of this embodiment, only theion conductor is formed within the via. In this case, a first electrodeis formed below the insulating material an in contact with the ionconductor and the second electrode is formed above the insulatingmaterial and in contact with the ion conductor. The configuration of thevia may be changed to alter (e.g., reduce) a contact area between one ormore of the electrodes and the ion conductor. Reducing thecross-sectional area of the interface between the ion conductor and theelectrode increases the efficiency of the device (change in electricalproperty per amount of power supplied to the device). In accordance withanother aspect of this embodiment, the via may extend through the lowerelectrode to reduce the interface area between the electrode and the ionconductor. In accordance with yet another aspect of this embodiment, aportion of the ion conductor may be removed from the via or the ionconductor material may be directionally deposited into only a portion ofthe via to further reduce an interface between an electrode and the ionconductor.

[0015] In accordance with another embodiment of the invention, aprogrammable device may be formed on a surface of a substrate.

[0016] In accordance with a further exemplary embodiment of theinvention, multiple bits of information are stored in a singleprogrammable structure. In accordance with one aspect of thisembodiment, a programmable structure includes a floating electrodeinterposed between two additional electrodes.

[0017] In accordance with yet another embodiment of the invention,multiple programmable devices are coupled together using a commonelectrode (e.g., a common anode or a common cathode).

[0018] In accordance with yet another embodiment of the invention,multiple programmable devices share a common electrode.

[0019] In accordance with yet a further exemplary embodiment of thepresent invention, a capacitance of a programmable structure is alteredby causing ions within an ion conductor of the structure to migrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] A more complete understanding of the present invention may bederived by referring to the detailed description and claims, consideredin connection with the figures, wherein like reference numbers refer tosimilar elements throughout the figures, and:

[0021]FIG. 1 is a cross-sectional illustration of a programmablestructure formed on a surface of a substrate in accordance with thepresent invention;

[0022]FIG. 2 is a cross-sectional illustration of a programmablestructure in accordance with an alternative embodiment of the presentinvention;

[0023]FIG. 3 is a current-voltage diagram illustrating current andvoltage characteristics of the device illustrated in FIG. 2 in an “on”and “off” state;

[0024]FIG. 4 is a cross-sectional illustration of a programmablestructure in accordance with yet another embodiment of the presentinvention;

[0025]FIG. 5 is a schematic illustration of a portion of a memory devicein accordance with an exemplary embodiment of the present invention;

[0026]FIG. 6 is a schematic illustration of a portion of a memory devicein accordance with an alternative embodiment of the present invention;

[0027]FIGS. 7 and 8 are a cross-sectional illustrations of aprogrammable structure having an ion conductor/electrode contactinterface formed about a perimeter of the ion conductor in accordancewith another embodiment of the present invention;

[0028]FIGS. 9 and 10 are a cross-sectional illustrations of aprogrammable structure having an ion conductor/electrode contactinterface formed about a perimeter of the ion conductor in accordancewith yet another embodiment of the present invention;

[0029]FIGS. 11 and 12 illustrate a programmable device having ahorizontal configuration in accordance with the present invention;

[0030] FIGS. 13-19 illustrate programmable device structures withreduced electrode/ion conductor interface surface area in accordancewith the present invention;

[0031]FIG. 20 illustrates a programmable device with a tapered ionconductor in accordance with the present invention;

[0032] FIGS. 21-24 illustrate a programmable device including a floatingelectrode in accordance with the present invention; and

[0033] FIGS. 25-29 illustrate common electrode programmable devicestructures in accordance with the present invention.

[0034] Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the dimensions of some of the elements inthe figures may be exaggerated relative to other elements to help toimprove understanding of embodiments of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0035] The present invention generally relates to microelectronicdevices. More particularly, the invention relates to programmablestructures or devices suitable for various integrated circuitapplications.

[0036]FIGS. 1 and 2 illustrate programmable microelectronic structures100 and 200 formed on a surface of a substrate 110 in accordance with anexemplary embodiment of the present invention. Structures 100 and 200include electrodes 120 and 130, an ion conductor 140, and optionallyinclude buffer or barrier layers 155 and/or 255.

[0037] Generally, structures 100 and 200 are configured such that when abias greater than a threshold voltage (V_(T)), discussed in more detailbelow, is applied across electrodes 120 and 130, the electricalproperties of structure 100 change. For example, in accordance with oneembodiment of the invention, as a voltage V≧V_(T) is applied acrosselectrodes 120 and 130, conductive ions within ion conductor 140 beginto migrate and form an electrodeposit (e.g., electrodeposit 160) at ornear the more negative of electrodes 120 and 130; such anelectrodeposit, however, is not required to practice the presentinvention. The term “electrodeposit” as used herein means any areawithin the ion conductor that has an increased concentration of reducedmetal or other conductive material compared to the concentration of suchmaterial in the bulk ion conductor material. As the electrodepositforms, the resistance between electrodes 120 and 130 decreases, andother electrical properties may also change. In the absence of anyinsulating barriers, which are discussed in more detail below, thethreshold voltage required to grow the electrodeposit from one electrodetoward the other and thereby significantly reduce the resistance of thedevice is approximately the redox potential of the system, typically afew hundred millivolts. If the same voltage is applied in reverse, theelectrodeposit will dissolve back into the ion conductor and the devicewill return to a high resistance state. In accordance with otherembodiments of the invention, application of an electric field betweenelectrodes 120 and 130 may cause ions dissolved within conductor 140 tomigrate and thus cause a change in the electrical properties of device100, without the formation of an electrodeposit. Structures 100 and 200may be used to store information and thus may be used in memorycircuits. For example, structure 100 or other programmable structures inaccordance with the present invention may suitably be used in memorydevices to replace DRAM, SRAM, PROM, EPROM, or EEPROM devices. Inaddition, programmable structures of the present invention may be usedfor other applications where programming or changing of electricalproperties of a portion of an electrical circuit are desired.

[0038] Substrate 110 may include any suitable material. For example,substrate 110 may include semiconductive, conductive, semiinsulative,insulative material, or any combination of such materials. In accordancewith one embodiment of the invention, substrate 110 includes aninsulating material 112 and a portion 114 including microelectronicdevices formed on a semiconductor substrate. Layers 112 and 114 may beseparated by additional layers (not shown) such as, for example, layerstypically used to form integrated circuits. Because the programmablestructures can be formed over insulating or other materials, theprogrammable structures of the present invention are particularly wellsuited for applications where substrate (e.g., semiconductor material)space is a premium.

[0039] Electrodes 120 and 130 may be formed of any suitable conductivematerial. For example, electrodes 120 and 130 may be formed of dopedpolysilicon material or metal.

[0040] In accordance with one exemplary embodiment of the invention, oneof electrodes 120 and 130 is formed of a material including a metal thatdissolves in ion conductor 140 when a sufficient bias (V≧V_(T)) isapplied across the electrodes (oxidizable electrode) and the otherelectrode is relatively inert and does not dissolve during operation ofthe programmable device (an indifferent electrode). For example,electrode 120 may be an anode during a write process and be comprised ofa material including silver that dissolves in ion conductor 140 andelectrode 130 may be a cathode during the write process and be comprisedof an inert material such as tungsten, nickel, molybdenum, platinum,metal silicides, and the like. Having at least one electrode formed of amaterial including a metal which dissolves in ion conductor 140facilitates maintaining a desired dissolved metal concentration withinion conductor 140, which in turn facilitates rapid and stableelectrodeposit 160 formation within ion conductor 140 or otherelectrical property change during use of structure 100 and/or 200.Furthermore, use of an inert material for the other electrode (cathodeduring a write operation) facilitates electrodissolution of anyelectrodeposit that may have formed and/or return of the programmabledevice to an erased state after application of a sufficient voltage.

[0041] During an erase operation, dissolution of any electrodeposit thatmay have formed preferably begins at or near the oxidizableelectrode/electrodeposit interface. Initial dissolution of theelectrodeposit at the oxidizable electrode/electrodeposit interface maybe facilitated by forming structure 100 such that the resistance of theat the oxidizable electrode/electrodeposit interface is greater than theresistance at any other point along the electrodeposit, particularly,the interface between the electrodeposit and the indifferent electrode.

[0042] One way to achieve relatively low resistance at the indifferentelectrode is to form the electrode of relatively inert, non-oxidizingmaterial such as platinum. Use of such material reduces formation ofoxides at the interface between ion conductor 140 and the indifferentelectrode as well as the formation of compounds or mixtures of theelectrode material and ion conductor 140 material, which typically havea higher resistance than ion conductor 140 or the electrode material.

[0043] Relatively low resistance at the indifferent electrode may alsobe obtained by forming a barrier layer between the oxidizable electrode(anode during a write operation), wherein the barrier layer is formed ofmaterial having a relatively high resistance. Exemplary high resistancematerials include layers (e.g., layer 155 and/or layer 255) of ionconducting material (e.g., Ag_(x)O, Ag_(x)S, Ag_(x)Se, Ag_(x)Te, wherex≧2, Ag_(y)I, where x≧1, CuI₂, CuO, CuS, CuSe, CuTe, GeO₂, or SiO₂)interposed between ion conductor 140 and a metal layer such as silver.Some of these materials have additional benefits as discussed in moredetail below.

[0044] Reliable growth and dissolution of an electrodeposit can also befacilitated by providing a roughened indifferent electrode surface(e.g., a root mean square roughness of greater than about 1 nm) at theelectrode/ion conductor interface. The roughened surface may be formedby manipulating film deposition parameters and/or by etching a portionof one of the electrode of ion conductor surfaces. During a writeoperation, relatively high electrical fields form about the spikes orpeaks of the roughened surface, and thus the electrodeposits are morelikely to form about the spikes or peaks. As a result, more reliable anduniform changes in electrical properties for an applied voltage acrosselectrodes 120 and 130 may be obtained by providing a roughed interfacebetween the indifferent electrode (cathode during a write operation) andion conductor 140.

[0045] Oxidizable electrode material may have a tendency to thermallydissolve or diffuse into ion conductor 140, particularly duringfabrication and/or operation of structure 100. The thermal diffusion isundesired because it may reduce the resistance of structure 100 and thusreduce the change of an electrical property during use of structure 100.

[0046] To reduce undesired diffusion of oxidizable electrode materialinto ion conductor 140 and in accordance with another embodiment of theinvention, the oxidizable electrode includes a metal intercalated in atransition metal sulfide or selenide material such as A_(x)(MB₂)_(1−x),where A is Ag or Cu, B is S or Se, M is a transition metal such as Ta,V, and Ti, and x ranges from about 0.1 to about 0.7. The intercalatedmaterial mitigates undesired thermal diffusion of the metal (Ag or Cu)into the ion conductor material, while allowing the metal to participatein the electrodeposit growth upon application of a sufficient voltageacross electrodes 120 and 130. For example, when silver in intercalatedinto a TaS₂ film, the TaS₂ film can include up to about 66.8 atomicpercent silver. The A_(x)(MB₂)_(1−x) material is preferably amorphous toprevent to prevent undesired diffusion of the metal though the material.The amorphous material may be formed by, for example, physical vapordeposition of a target material comprising A_(x)(MB₂)_(1−x).

[0047] α-AgI is another suitable material for the oxidizable electrode,as well as the indifferent electrode. Similar to the A_(x)(MB₂)_(1−x)material discussed above, α-AgI can serve as a source of Ag duringoperation of structure 100—e.g., upon application of a sufficient bias,but the silver in the AgI material does not readily thermally diffuseinto ion conductor 140. AgI has a relatively low activation energy forconduction of electricity and does not require doping to achieverelatively high conductivity. When the oxidizable electrode is formed ofAgI, depletion of silver in the AgI layer may arise during operation ofstructure 100, unless excess silver is provided to the electrode. Oneway to provide the excess silver is to form a silver layer adjacent theAgI layer as discussed above when AgI is used as a buffer layer. The AgIlayer (e.g., layer 155 and/or 255) reduces thermal diffusion of Ag intoion conductor 140, but does not significantly affect conduction of Agduring operation of structure 100. In addition, use of AgI increases theoperational efficiency of structure 100 because the AgI mitigatesnon-Faradaic conduction (conduction of electrons that do not participatein the electrochemical reaction).

[0048] Other materials suitable for buffer layers 155 and/or 255 includeGeO₂ and SiO_(x). Amorphous GeO₂ is relatively porous an will “soak up”silver during operation of device 100, but will retard the thermaldiffusion of silver to ion conductor 140, compared to structures ordevices that do not include a buffer layer. When ion conductor 140includes germanium, GeO₂ may be formed by exposing ion conductor 140 toan oxidizing environment at a temperature of about 300° C. to about 800°C. or by exposing ion conductor 140 to an oxidizing environment in thepresence of radiation having an energy greater than the band gap of theion conductor material. The GeO₂ may also be deposited using physicalvapor deposition (from a GeO₂ target) or chemical vapor deposition (fromGeH₄ and an O₂).

[0049] Buffer layers can also be used to increase a “write voltage” byplacing the buffer layer (e.g., GeO₂ or SiO_(x)) between ion conductor140 and the indifferent electrode. The buffer material allows metal suchas silver to diffuse though the buffer and take part in theelectrochemical reaction.

[0050] In accordance with one embodiment of the invention, at least oneelectrode 120 and 130 is formed of material suitable for use as aninterconnect metal. For example, electrode 130 may form part of aninterconnect structure within a semiconductor integrated circuit. Inaccordance with one aspect of this embodiment, electrode 130 is formedof a material that is substantially insoluble in material comprising ionconductor 140. Exemplary materials suitable for both interconnect andelectrode 130 material include metals and compounds such as tungsten,nickel, molybdenum, platinum, metal silicides, and the like.

[0051] Layers 155 and/or 255 may also include a material that restrictsmigration of ions between conductor 140 and the electrodes. Inaccordance with exemplary embodiments of the invention, a barrier layerincludes conducting material such as titanium nitride, titaniumtungsten, a combination thereof, or the like. The barrier may beelectrically indifferent, i.e., it allows conduction of electronsthrough structure 100 or 200, but it does not itself contribute ions toconduction through structure 200. An electrically indifferent barriermay reduce undesired dendrite growth during operation of theprogrammable device, and thus may facilitate an “erase” or dissolutionof electrodeposit 160 when a bias is applied which is opposite to thatused to grow the electrodeposit. In addition, use of a conductingbarrier allows for the “indifferent” electrode to be formed ofoxidizable material because the barrier prevents diffusion of theelectrode material to the ion conductor.

[0052] Ion conductor 140 is formed of material that conducts ions uponapplication of a sufficient voltage. Suitable materials for ionconductor 140 include glasses and semiconductor materials. In oneexemplary embodiment of the invention, ion conductor 140 is formed ofchalcogenide material.

[0053] Ion conductor 140 may also suitably include dissolved conductivematerial. For example, ion conductor 140 may comprise a solid solutionthat includes dissolved metals and/or metal ions. In accordance with oneexemplary embodiment of the invention, conductor 140 includes metaland/or metal ions dissolved in chalcogenide glass. An exemplarychalcogenide glass with dissolved metal in accordance with the presentinvention includes a solid solution of As_(x)S_(1−x)Ag,Ge_(x)Se_(1−x)Ag, Ge_(x)S_(1−x)Ag, As_(x)S_(1−x)—Cu, Ge_(x)Se_(1−x)—Cu,Ge_(x)S_(1−x)—Cu, where x ranges from about 0.1 to about 0.5 otherchalcogenide materials including silver, copper, zinc, combinations ofthese materials, and the like. In addition, conductor 140 may includenetwork modifiers that affects mobility of ions through conductor 140.For example, materials such as metals (e.g., silver), halogens, halides,or hydrogen may be added to conductor 140 to enhance ion mobility andthus increase erase/write speeds of the structure.

[0054] A solid solution suitable for use as ion conductor 140 may beformed in a variety of ways. For example, the solid solution may beformed by depositing a layer of conductive material such as metal overan ion conductive material such as chalcogenide glass and exposing themetal and glass to thermal and/or photo dissolution processing. Inaccordance with one exemplary embodiment of the invention, a solidsolution of As₂S₃—Ag is formed by depositing As₂S₃ onto a substrate,depositing a thin film of Ag onto the As₂S₃, and exposing the films tolight having energy greater than the optical gap of the As₂S_(3,)—e.g.,light having a wavelength of less than about 500 nanometers. If desired,network modifiers may be added to conductor 140 during deposition ofconductor 140 (e.g., the modifier is in the deposited material orpresent during conductor 140 material deposition) or after conductor 140material is deposited (e.g., by exposing conductor 140 to an atmosphereincluding the network modifier).

[0055] In accordance with another embodiment of the invention, a solidsolution may be formed by depositing one of the constituents onto asubstrate or another material layer and reacting the first constituentwith a second constituent. For example, germanium (preferably amorphous)may be deposited onto a portion of a substrate and the germanium may bereacted with H₂Se to form a Ge—Se glass. Similarly, As can be depositedand reacted with the H₂Se gas, or arsenic or germanium can be depositedand reacted with H₂S gas. Silver or other metal can then be added to theglass as described above.

[0056] In accordance with one aspect of this embodiment, a solidsolution ion conductor 140 is formed by depositing sufficient metal ontoan ion conductor material such that a portion of the metal can bedissolved within the ion conductor material and a portion of the metalremains on a surface of the ion conductor to form an electrode (e.g.,electrode 120). In accordance with alternative embodiments of theinvention, solid solutions containing dissolved metals may be directlydeposited onto substrate 110 and the electrode then formed overlying theion conductor.

[0057] An amount of conductive material such as metal dissolved in anion conducting material such as chalcogenide may depend on severalfactors such as an amount of metal available for dissolution and anamount of energy applied during the dissolution process. However, when asufficient amount of metal and energy are available for dissolution inchalcogenide material using photodissolution, the dissolution process isthought to be self limiting, substantially halting when the metalcations have been reduced to their lowest oxidation state. In the caseof As₂S₃—Ag, this occurs at Ag₄As₂S₃=2Ag₂S+As₂S, having a silverconcentration of about 44 atomic percent. If, on the other hand, themetal is dissolved in the chalcogenide material using thermaldissolution, a higher atomic percentage of metal in the solid solutionmay be obtained, provided a sufficient amount of metal is available fordissolution.

[0058] In accordance with a further embodiment of the invention, thesolid solution is formed by photodissolution to form a macrohomogeneousternary compound and additional metal is added to the solution usingthermal diffusion (e.g., in an inert environment at a temperature ofabout 85° C. to about 150° C.) to form a solid solution containing, forexample, about 30 to about 50, and preferably about 34 atomic percentsilver. Ion conductors having a metal concentration above thephotodissolution solubility level facilitates formation ofelectrodeposits that are thermally stable at operating temperatures(typically about 85° C. to about 150° C.) of devices 100 and 200.Alternatively, the solid solution may be formed by thermally dissolvingthe metal into the ion conductor at the temperature noted above;however, solid solutions formed exclusively from photodissolution arethought to be less homogeneous than films having similar metalconcentrations formed using photodissolution and thermal dissolution.

[0059] Ion conductor 140 may also include a filler material, which fillsinterstices or voids. Suitable filler materials include non-oxidizableand non-silver based materials such as a non-conducting, immisciblesilicon oxide and/or silicon nitride, having a cross-sectional dimensionof less than about 1 nm, which do not contribute to the growth of anelectrodeposit. In this case, the filler material is present in the ionconductor at a volume percent of up to about 5 percent to reduce alikelihood that an electrodeposit will spontaneously dissolve into thesupporting ternary material as the device is exposed to elevatedtemperature, which leads to more stable device operation withoutcompromising the performance of the device. Ion conductor 140 may alsoinclude filler material to reduce an effective cross-sectional area ofthe ion conductor. In this case, the concentration of the fillermaterial, which may be the same filler material described above buthaving a cross-sectional dimension up to about 50 nm, is present in theion conductor material at a concentration of up to about 50 percent byvolume. The filler material may also include metal such as silver orcopper to fill the voids in the ion conductor material.

[0060] In accordance with one exemplary embodiment of the invention, ionconductor 140 includes a germanium-selenide glass with silver diffusedin the glass. Germanium selenide materials are typically formed fromselenium and Ge(Se)_(4/2) tetrahedra that may combine in a variety ofways. In a Se-rich region, Ge is 4-fold coordinated and Se is 2-foldcoordinated, which means that a glass composition nearGe_(0.20)Se_(0.80) will have a mean coordination number of about 2.4.Glass with this coordination number is considered by constraint countingtheory to be optimally constrained and hence very stable with respect todevitrification. The network in such a glass is known to self-organizeand become stress-free, making it easy for any additive, e.g., silver,to finely disperse and form a mixed-glass solid solution. Accordingly,in accordance with one embodiment of the invention, ion conductor 140includes a glass having a composition of Ge_(0.17)Se_(0.83) toGe_(0.25)Se_(0.75).

[0061] The composition and structure of ion conductor 140 material oftendepends on the starting or target material used to form the conductor.Generally, it is desired to form a homogenous material layer forconductor 140 to facilitate reliable and repeatable device performance.In accordance with one embodiment of the invention, a target forphysical vapor deposition of material suitable for ion conductor 140 isformed by selecting a proper ampoule, preparing the ampoule, maintainingproper temperatures during formation of the glass, slow rocking thecomposition, and quenching the composition.

[0062] Volume and wall thickness are important factors for considerationin selecting an ampoule for forming glass. The wall thickness must bethick enough to withstand gas pressures that arise during the glassformation process and are preferably thin enough to facilitate heatexchange during the formation process. In accordance with exemplaryembodiment of the invention, quartz ampoules with a wall thickness ofabout 1 mm are used to form Se and Te based chalcogenide glasses,whereas quartz ampoules with a wall thickness of about 1.5 mm are usedto form sulfur-based chalcogenide glasses. In addition, the volume ofthe ampoule is preferably selected such that the volume of the ampouleis about five times greater than the liquid glass precursor material.

[0063] Once the ampoule is selected, the ampoule is prepared for glassformation, in accordance with one embodiment of the invention, bycleaning the ampoule with hydrofluoric acid, ethanol and acetone, dryingthe ampoule for at least 24 hours at about 120° C., evacuating theampoule and heating the ampoule until the ampoule turns a cherry redcolor and cooling the ampoule under vacuum, filling ampoule with chargeand evacuating the ampoule, heating the ampoule while avoiding meltingof the constituents to desorb any remaining oxygen, and sealing theampoule. This process reduces oxygen contamination, which in turnpromotes macrohomogeneous growth of the glass.

[0064] The melting temperature of the glass formation process depends onthe glass material. In the case of germanium-based glasses, sufficienttime for the chalcogen to react at low temperature with all availablegermanium is desired to avoid explosion at subsequent elevatedtemperatures (the vapor pressure of Se at 920° C. is 10 ATM. and 20 ATM.for S at 720° C.). To reduce the risk of explosion, the glass formationprocess begins by ramping the ampoule temperature to about 300° C. forselenium-based glasses (about 200° C. for sulfur-based glasses) over theperiod of about an hour and maintaining this temperature for about 12hours. Next, the temperature is elevated slowly (about 0.5° C./min) upto a temperature about 50° C. higher than the liquidus temperature ofthe material and the ampoule remains at about this temperature for about12 hours. The temperature is then elevated to about 940° C. to ensuremelting of all non-reacted germanium for Se-based glasses or about 700°C. for S-based glasses. The ampoule should remain at this elevatedtemperature for about 24 hours.

[0065] The melted glass composition is preferably slow rocked at a rateof about 20/minute at least about six hours to increase the homogeneityof the glass.

[0066] Quenching is preferably performed from a temperature at which thevapors and the liquid are in an equilibrium to produce vitrification ofthe desired composition. In this case, the quenching temperature isabout 50° C. over the liquidus temperature of the glass material.Chalcogenide-rich glasses include a range of concentrations in whichunder-constrained and over-constrained glasses exist. In cases where theglass composition coordinated number is far from the optimalcoordination (e.g., coordination numbers of about 2.4 for Ge—Se systems)the quenching rate has to be fast enough in order to ensurevitrification, e.g., quenching in ice-water or an even stronger coolantsuch as a mixture of urea and ice-water. In the case of optimallycoordinated glasses, quenching can be performed in air at about 25° C.

[0067] In accordance with one exemplary embodiment of the invention, atleast a portion of structure 100 is formed within a via of an insulatingmaterial 150. Forming a portion of structure 100 within a via of aninsulating material 150 may be desirable because, among other reasons,such formation allows relatively small structures, e.g., on the order of10 nanometers, to be formed. In addition, insulating material 150facilitates isolating various structures 100 from other electricalcomponents.

[0068] Insulating material 150 suitably includes material that preventsundesired diffusion of electrons and/or ions from structure 100. Inaccordance with one embodiment of the invention, material 150 includessilicon nitride, silicon oxynitride, polymeric materials such aspolyimide or parylene, or any combination thereof.

[0069] A contact 165 may suitably be electrically coupled to one or moreelectrodes 120,130 to facilitate forming electrical contact to therespective electrode. Contact 165 may be formed of any conductivematerial and is preferably formed of a metal such as aluminum, aluminumalloys, tungsten, or copper.

[0070] In accordance with one embodiment of the invention, structure 100is formed by forming electrode 130 on substrate 110. Electrode 130 maybe formed using any suitable method such as, for example, depositing alayer of electrode 130 material, patterning the electrode material, andetching the material to form electrode 130. Insulating layer 150 may beformed by depositing insulating material onto electrode 130 andsubstrate 110 and forming vias in the insulating material usingappropriate patterning and etching processes. Ion conductor 140 andelectrode 120 may then be formed within insulating layer 150 bydepositing ion conductor 140 material and electrode 120 material withinthe via. Such ion conductor and electrode material deposition may beselective—i.e., the material is substantially deposited only within thevia, or the deposition processes may be relatively non-selective. If oneor more non-selective deposition methods are used, any excess materialremaining on a surface of insulating layer 150 may be removed using, forexample, chemical mechanical polishing and/or etching techniques.Barrier layers 155 and/or 255 may similarly be formed using any suitabledeposition and/or etch processes.

[0071] Information may be stored using programmable structures of thepresent invention by manipulating one or more electrical properties ofthe structures. For example, a resistance of a structure may be changedfrom a “0” or off state to a “1” or on state during a suitable writeoperation. Similarly, the device may be changed from a “1” state to a“0” state during an erase operation. In addition, as discussed in moredetail below, the structure may have multiple programmable states suchthat multiple bits of information are stored in a single structure.

Write Operation

[0072]FIG. 3 illustrates current-voltage characteristics of aprogrammable structure (e.g. structure 200) in accordance with thepresent invention. In the illustrated embodiment, via diameter, D, isabout 4 microns, conductor 140 is about 35 nanometers thick and formedof Ge₃Se₇—Ag (near As₈Ge₃Se₇), electrode 130 is indifferent and formedof nickel, electrode 120 is formed of silver, and barrier 255 is anative nickel oxide. As illustrated in FIG. 3, current through structure200 in an off state (curve 310) begins to rise upon application of abias of over about one volt; however, once a write step has beenperformed (i.e., an electrodeposit has formed), the resistance throughconductor 140 drops significantly (i.e., to about 200 ohms), illustratedby curve 320 in FIG. 3. As noted above, when electrode 130 is coupled toa more negative end of a voltage supply, compared to electrode 120, anelectrodeposit begins to form near electrode 130 and grow towardelectrode 120. An effective threshold voltage (i.e., voltage required tocause growth of the electrodeposit and to break through barrier 255,thereby coupling electrodes 320, 330 together is relatively high becauseof barrier 255. In particular, a voltage V≧V_(T) must be applied tostructure 200 sufficient to cause electrons to tunnel through barrier255 (when barrier 255 comprises an insulating layer) to form theelectrodeposit and to overcome the barrier (e.g., by tunneling throughor leakage) and conduct through conductor 140 and at least a portion ofbarrier 255.

[0073] In accordance with alternate embodiments of the invention, whereno insolating barrier layer is present, an initial “write” thresholdvoltage is relatively low because no insulative barrier is formedbetween, for example, ion conductor 140 and either of the electrodes120, 130.

Read Operation

[0074] A state of the device (e.g., 1 or 0) may be read, withoutsignificantly disturbing the state, by, for example, applying a forwardor reverse bias of magnitude less than a voltage threshold (about 1.4 Vfor a structure illustrated in FIG. 3) for electrodeposition or by usinga current limit which is less than or equal to the minimum programmingcurrent (the current which will produce the highest of the on resistancevalues). A current limited (to about 1 milliamp) read operation is shownin FIG. 3. In this case, the voltage is swept from 0 to about 2 V andthe current rises up to the set limit (from 0 to 0.2 V), indicating alow resistance (ohmic/linear current-voltage) “on” state. Another way ofperforming a non-disturb read operation is to apply a pulse, with arelatively short duration, which may have a voltage higher than theelectrochemical deposition threshold voltage such that no appreciableFaradaic current flows, i.e., nearly all the current goes topolarizing/charging the device and not into the electrodepositionprocess.

Erase Operation

[0075] A programmable structure (e.g., structure 200) may suitably beerased by reversing a bias applied during a write operation, wherein amagnitude of the applied bias is equal to or greater than the thresholdvoltage for electrodeposition in the reverse direction. In accordancewith an exemplary embodiment of the invention, a sufficient erasevoltage (V≧V_(T)) is applied to structure 200 for a period of time whichdepends on the strength of the initial connection but is typically lessthan about 1 millisecond to return structure 200 to its “off” statehaving a resistance well in excess of a million ohms. In cases where theprogrammable structure does not include a barrier between conductor 140and electrode 120, a threshold voltage for erasing the structure is muchlower than a threshold voltage for writing the structure because, unlikethe write operation, the erase operation does not require electrontunneling through a barrier or barrier breakdown.

Control of Operational Parameters

[0076] The concentration of conductive material in the ion conductor canbe controlled by applying a bias across the programmable device. Forexample, metal such as silver may be taken out of solution by applying anegative voltage in excess of the reduction potential of the conductivematerial. Conversely, conductive material may be added to the ionconductor (from one of the electrodes) by applying a bias in excess ofthe oxidation potential of the material. Thus, for example, if theconductive material concentration is above that desired for a particulardevice application, the concentration can be reduced by reverse biasingthe device to reduce the concentration of the conductive material.Similarly, metal may be added to the solution from the oxidizableelectrode by applying a sufficient forward bias. Additionally, it ispossible to remove excess metal build up at the indifferent electrode byapplying a reverse bias for an extended time or an extended bias overthat required to erase the device under normal operating conditions.Control of the conductive material may be accomplished automaticallyusing a suitable microprocessor.

[0077] This technique may also be used to form one of the electrodesfrom material within the ion conductor material. For example, silverfrom the ion conductor may be plated out to form the oxidizableelectrode. This allows the oxidizable electrode to be formed after thedevice is fully formed and thus mitigates problems associated withconductive material diffusing from the oxidizable electrode duringmanufacturing of the device.

[0078] As noted above, in accordance with yet another embodiment of theinvention, multiple bits of data may be stored within a singleprogrammable structure by controlling an amount of electrodeposit whichis formed during a write process. An amount of electrodeposit that formsduring a write process depends on a number of coulombs or chargesupplied to the structure during the write process, and may becontrolled by using a current limit power source. In this case, aresistance of a programmable structure is governed by Equation 1, whereR_(on) is the “on” state resistance, V_(T) is the threshold voltage forelectrodeposition , and I_(LIM) is the maximum current allowed to flowduring the write operation. $\begin{matrix}{R_{on} = \frac{V_{T}}{I_{LIM}}} & {{Equation}\quad 1}\end{matrix}$

[0079] In practice, the limitation to the amount of information storedin each cell will depend on how stable each of the resistance states iswith time. For example, if a structure is with a programmed resistancerange of about 3.5 kΩ and a resistance drift over a specified time foreach state is about ±250Ω, about 7 equally sized bands of resistance (7states) could be formed, allowing 3 bits of data to be stored within asingle structure. In the limit, for near zero drift in resistance in aspecified time limit, information could be stored as a continuum ofstates, i.e., in analog form.

[0080] A portion of an integrated circuit 402, including a programmablestructure 400, configured to provide additional isolation fromelectronic components is illustrated in FIG. 4. In accordance with anexemplary embodiment of the present invention, structure 400 includeselectrodes 420 and 430, an ion conductor 440, a contact 460, and anamorphous silicon diode 470, such as a Schottky or p-n junction diode,formed between contact 460 and electrode 420. Rows and columns ofprogrammable structures 400 may be fabricated into a high densityconfiguration to provide extremely large storage densities suitable formemory circuits. In general, the maximum storage density of memorydevices is limited by the size and complexity of the column and rowdecoder circuitry. However, a programmable structure storage stack canbe suitably fabricated overlying an integrated circuit with the entiresemiconductor chip area dedicated to row/column decode, senseamplifiers, and data management circuitry (not shown) since structure400 need not use any substrate real estate. In this manner, storagedensities of many gigabits per square centimeter can be attained usingprogrammable structures of the present invention. Utilized in thismanner, the programmable structure is essentially an additive technologythat adds capability and functionality to existing semiconductorintegrated circuit technology.

[0081]FIG. 5 schematically illustrates a portion of a memory deviceincluding structure 400 having an isolating p-n junction 470 at anintersection of a bit line 510 and a word line 520 of a memory circuit.FIG. 6 illustrates an alternative isolation scheme employing atransistor 610 interposed between an electrode and a contact of aprogrammable structure located at an intersection of a bit line 610 anda word line 620 of a memory device.

[0082] FIGS. 7-10 illustrate programmable devices in accordance withanother embodiment of the invention. The devices illustrated in FIGS.7-10 have an electrode (e.g., the cathode during a write process) with asmaller cross sectional area in contact with the ion conductor comparedto the devices illustrated in FIGS. 1-2 and 4. The smaller electrodeinterface area is thought to increase the efficiency and endurance ofthe device because an increased percentage of ions in the solid solutionare able to take part in the electrodeposit formation process. Thus anycathode plating from ions that do not participate in the electrodepositprocess is reduced.

[0083]FIGS. 7 and 8 illustrate a cross sectional and a top cut-away viewof a programmable device 700 including an indifferent electrode 710, anoxidizable electrode 720, and an ion conductor 730 former overlying aninsulating layer 740 such as silicon oxide, silicon nitride, or thelike.

[0084] Structure 700 is formed by depositing an indifferent electrodematerial layer and an insulating layer 750 overlying insulating layer740. A via is then formed through layer 750 and electrode material layer710, using an anisotropic etch process (e.g., reactive ion etching orion milling) such that the via extends to and/or through a portion oflayer 740. The via is then filled with ion conductor material and issuitably doped to form a solid solution as described herein. Any excession conductor material is removed from the surface of layer 750 andelectrode 730 is formed, for example using a deposition and etchprocess. In this case, the indifferent electrode (cathode during writeprocess) area in contact with ion conductor 730 is the surface area ofelectrode 710 about the perimeter of conductor 730, rather than the areaunderlying the ion conductor, as illustrated in FIGS. 1-2 and 4.

[0085]FIGS. 9 and 10 illustrate a programmable device 900 having anindifferent electrode 910, an oxidizable electrode 920, an ion conductor930 and insulating layers 940 and 950 in accordance with yet anotherembodiment of the invention. Structure 900 is similar to structure 700,except that once a via is formed through layer 750, an isotropic etchprocess (e.g., chemical or plasma) is employed to form the via throughelectrode 910, such that a sloped intersection between an ion conductor930 and electrode 910 is formed.

[0086]FIGS. 11 and 12 illustrate another programmable device 1100, witha reduced electrode/ion conductor interface, in accordance with thepresent invention. Structure 1100 includes electrodes 1110 and 1120 andan ion conductor 1130, formed on a surface of an insulating material1140, rather than within a via as discussed above. In this case, theprogrammable structure is formed by defining an ion conductor 1130patter on a surface of insulating material 1140 (e.g., using depositionand etch techniques) and forming electrodes 1110 and 1120, such that theelectrodes each contact a portion of the ion conductor. In the case ofthe illustrated embodiment, the electrodes are formed overlying and incontact with both a portion of the ion conductor and the insulatingmaterial. Although the thickness of the layers may be varied inaccordance with specific applications of the device, in a preferredembodiment of the invention, the thickness of the ion conductor andelectrode films is about 1 nm to about 100 nm. Sub-lithographic lateraldimensions of portions of the device may be obtained by overexposingphotoresist used to pattern the portions and/or over etching the filmlayer.

[0087]FIG. 13 illustrates a device 1300 in accordance with yet anotherembodiment of the invention. Structure 1300 is similar to the devicesillustrated in FIG. 7 and 8, except that the cross-sectional area of theion conductor that is in contact with the electrodes is reduced byfilling a portion of a via with non-ion conductor material, rather thanetching through an electrode layer.

[0088] Structure 1300 includes electrodes 1310 and 1320 and an ionconductor 1330 formed within an insulating layer 1340. In this case, ionconductor 1330 is formed by creating a trench within insulating layer1340, the trench having a diameter indicated by D2. The trench is thenfilled using, for example, interference lithography techniques orconformally lining the via with insulating material and using ananisotropic etch process to remove some of the insulating material,leaving a via with a diameter of D3. Structure 1300 formed using thistechnique may have a ion conductor cross sectional area as small asabout 10 nm in contact with electrodes 1310 and 1320.

[0089] FIGS. 14-17 illustrate another embodiment of the invention, wherethe cross sectional area of the ion conductor/electrode interface isrelatively small. Structure 1400, illustrated in FIG. 14, includeselectrodes 1410 and 1420 and an ion conductor 1430. Structure 1400 isformed in a manner similar to structure 700, except that the ionconductor material is deposited conformally, using, for example chemicalvapor deposition or physical vapor deposition, into a trench, and thetrench is not filled with the ion conductor material.

[0090] Structure 1500 is similar to structure 1400, except that an ionconductor 1530 is formed by etching a portion of ion conductor 1430,such that a via 1540 is formed through to electrode 1410. Structure 1600is similar to structure 1500 and is formed by conformally depositing theion conductor material as described above and then removing the ionconductor material from a surface of insulating material 1450 prior todepositing electrode 1420 material. Finally, structure 1700 may beformed by selectively deposing the ion conductor 1730 material into onlya portion of the trench formed in insulating material 1450 (e.g., usingangled deposition and/or shadowing techniques), removing any excess ionconductor material on the surface of insulator 1450, and forming anelectrode 1720 overlying the insulator and in contact with ion conductor1730.

[0091]FIGS. 18 and 19 illustrate yet another embodiment of theinvention, where a pillar or wall within a trench is used to reduce across-sectional area of the interface between the ion conductor and oneor more electrodes. Structure 1800, illustrated in FIG. 18, includeselectrodes 1810 and 1820 and an ion conductor 1830 formed within aninsulating layer 1840. In addition, structure 1800 includes a pillar1850 of insulating material (e.g., insulating material used to formlayer 1840), formed within a trench within layer 1840. Structure 1800may be formed using the shadowed deposition technique discussed above.Structure 1900 is similar to structure 1800, except structure 1900includes a partial pillar 1950 and an ion conductor 1930, which fillsthe remaining portion of the formed trench.

[0092]FIG. 20 illustrates yet another structure 2000 in accordance withthe present invention. Structure 2000 includes electrodes 2010 and 2020and an ion conductor 2030 formed within an insulating layer 2040.Structure 2000 is formed using an anisotropic or a combination of ananisotropic and an isotropic etch processes to form a tapered via. Ionconductor 2030 is then formed within the trench using techniquespreviously described.

[0093] FIGS. 21-24 illustrate programmable devices in accordance withyet another embodiment of the invention. The structures illustrated inFIGS. 21-24 include a floating electrode, which allows multiple bits ofinformation to be stored within a single programmable device.

[0094] Structure 2100 includes a first electrode 2110, a second,floating electrode 2120, a third electrode 2130, ion conductor portions2140 and 2150, which may all be formed on a substrate or wholly orpartially formed within a via as described above. Although structure2100 is illustrated in a vertical configuration, the structure may beformed in a horizontal configuration, similar to structure 1100. Inaccordance with one aspect of this embodiment, the first and thirdelectrodes are formed of an indifferent electrode and the secondelectrode is formed of an oxidizable electrode material. Alternatively,the first and third electrodes may be formed of oxidizable electrodematerial and the second, floating electrode may be formed of anindifferent electrode material. In either case, the structure includestwo “half cells,” where each half cell functions as a programmabledevice described above in connection with FIG. 1. Each half cell ispreferably configured such that the resistance of one half cell differsfrom the resistance of the other half cell when both cells are in anerased state.

[0095] In the case when floating electrode 2120 is formed of oxidizableelectrode material, bits of data may be stored as follows. The overallimpedance of structure 2100 is approximately equal to the resistance ofportions 2140 and 2150. When no electrodeposit is formed within eitherportion, this high resistance state may be represent by the state 00.When a voltage is applied to structure 2100, such that electrode 2130 ispositive relative to electrode 2110 and the applied bias is greater thatthe threshold voltage required to form an electrodeposit in portion2140, an electrodeposit 2160 will form through conductor portion 2140from electrode 2110 toward floating electrode 2120 as illustrated inFIG. 22. Under this condition, an electrodeposit will not form withinconductor portion 2150 because portion 2150 is under a reverse biascondition and thus will not support growth of an electrodeposit. Thegrowth of the electrodeposit will change the impedance of portion 2140from Z₁ to Z₁′, thus changing the overall impedance of structure 2100,which may be represent by the state 01. The current level used to formelectrodeposit 2160 should be selected such that it is sufficiently low,allowing the electrodeposit to be dissolved upon application of asufficient reverse bias. A third state may be formed by reversing thepolarity of the applied bias across electrodes 2110 and 2130, such thatmost of the voltage drop occurs across the high resistance ion conductorportion 2150 and formation of an electrodeposit 2170 begins, asillustrated in FIG. 23, without causing electrodeposit 2160 to dissolve.The impedance of portion 2150 changes from Z₂ to Z₂′, and the overallimpedance of structure 2100 is Z₁′ plus Z₂′, which may be represented bythe state 11. Once both half cells are in the write state,electrodeposit 2160 and/or 2170 may be dissolved by applying asufficient bias across one or both of the half cells. Electrodeposit2170 can be erased, for example, by sufficiently negatively biasingelectrode 2130 with respect to electrode 2110, which may be representedby a state 00. The four possible states, along with the current limitused to form the state, are represented in table 1 below. TABLE 1Current State/ Seq # Polarity limit Z half-cell 1 Z half-cell 2 value 1Sub-threshold Zero Z₁ Z₂ 00 2 Upper + Lower − Low Z₁′ Z₂ 01 3 Upper −Lower + Low Z₁′ Z₂′ 11 4 Upper − Lower + High Z₁ Z₂′ 10

[0096] Structure 2100 can be changed to 11 from state 10 by applying alow current limit bias to grow electrodeposit 2150 in portion 2140.Similarly, structure 2100 can be changed from state 11 to state 01 bydissolving electrodeposit 2170 by applying a relatively high currentlimit bias such that upper electrode 2130 is positive with respect tolower electrode 2110. Finally, structure 2100 can be returned to state00 using a short current pulse to thermally dissolve electrodeposit2160, using a current which is high enough to cause localized heating ofthe electrodeposit. This will increase the metal concentration in thehalf-cell but this excess metal can be removed electrically from thecell by plating it back onto the floating electrode. This sequence issummarized in table 2 below. TABLE 2 Current State/ Seq # Polarity limitZ half-cell 1 Z half-cell 2 value 4 Existing state — Z₁ Z₂′ 10 5 Upper +Lower − Low Z₁′ Z₂′ 11 6 Upper + Lower − High Z₁′ Z₂ 01 7 Upper + Lower− Thermal Z₁ Z₂ 00

[0097] Other write and erase sequences are also possible (as are otherdefinitions of the various states represented by the half-cellimpedances). For example, it is possible to go from state 00 to eitherstate 01 or state 10, depending on the write polarity chosen. Similarly,it is possible to go from state 11 to either state 10 or state 01. It isalso possible to go from state 11 to state 00 by the application of acurrent pulse (in either direction) which is high and short enough tothermally dissolve the electrodeposits in both half-cellssimultaneously.

[0098] In addition to storing information in digital form, structure2100 can also be used as a noise-tolerant, low energy anti-fuse elementfor use in field programmable gate arrays (FPGAs) and field configurablecircuits and systems. Most physical anti-fuse technologies require largecurrents and voltages to make a permanent connection. The need for suchhigh energy state-switching stimuli is generally considered to besomewhat beneficial as this reduces the likelihood of the anti-fuseaccidentally forming a connection in electrically noisy situations.However, the use of high voltages and large currents on chip represent asignificant problem as all components in the programming circuits aretypically sized accordingly and the high energy consumption reducesbattery life in portable systems.

[0099] FIGS. 25-29 illustrate structures in accordance with anotherembodiment of the invention in which multiple programmable devicesinclude a common electrode (e.g., the devices share a common anode orcathode. Forming structures in which multiple structures share a commonelectrode is advantageous because such structures allow a higher densityof cells to be formed on a given substrate surface area.

[0100]FIGS. 25 and 26 illustrate a structure 2500, having a horizontalconfiguration and a common electrode. Structure 2500 includes anelectrical connector 2510 coupled to a common surface electrode 2520,electrodes 2530 and 2540, and ion conductor portions 2550 and 2560overlying an insulating layer 2170. Structure 2500 may be used to formword and bit lines as described above by forming a row of electrodes(e.g., anodes) coupled to conductor 2510, and columns of oppositely biaselectrodes (e.g., cathodes) running perpendicular to electrodes 2520. Aconductive plug, formed of any suitably conducting material can be usedto electrically couple electrode 2520 to conductor 2510. Althoughillustrated with a horizontal configuration, common electrode structuresin accordance with this embodiment may be formed using structures havinga vertical configuration as described herein.

[0101]FIGS. 27 and 28 illustrate additional structures 2700 and 2800having a common electrode shared between two or more devices Structures2700 and 2800 include a common electrode, electrodes 2720 and 2725, ionconductors 2730,2735 and 2830, 2835 respectively, and insulating layers2740 and 2750. Structures 2700 and 2800 may be formed using techniquesdescribed above in connection with FIGS. 15 and 16—e.g., by conformallydepositing ion conductor material within a trench of an insulatinglayer. In accordance with another embodiment of the invention,directional deposition may be used to form a structure similar tostructure 1700. Structures 2700 and 2800 each include two programmabledevices including common electrode 2710 an ion conductor (e.g.,conductor 2735) and another electrode (e.g., electrode 2725). Dielectricmaterial 2750 is an insulating material that does not interfere withsurface electrodeposit growth, such as silicon oxides, silicon nitrides,and the like.

[0102]FIG. 29 illustrates a structure 2900 including multipleprogrammable devices 2902-2916 formed about a common electrode 2920.Each of the devices 2902-2916 may be formed using the method describedabove in connection with FIG. 21. In the embodiment illustrated in FIG.29, each of electrodes 2930-2936 and 2938-2944 may be coupled togetherin a direction perpendicular to the direction of common electrode 2920,such that electrode 2920 forms a bit line and electrodes 2930-2936 andelectrodes 2938-2944 form word lines. Structure 2900 may operate and beprogrammed in a manner similar to structure 2100 described above.

[0103] In accordance with other embodiments of the present invention, aprogrammable structure or device stores information by storing a chargeas opposed to growing an electrodeposit. A capacitance of a structure ordevice is altered by applying a bias across electrodes of the devicesuch that positively charged ions migrate toward one of the electrodes.If the applied bias is less that a write threshold voltage, no shortwill form between the electrodes. Capacitance of the structure changesas a result of the ion migration. When the applied bias is removed, themetal ions tend to diffuse away from the electrode or a barrierproximate the electrode. However, an interface between an ion conductorand a barrier is generally imperfect and includes defects capable oftrapping ions. Thus, at least a portion of ions remain at or proximatean interface between a barrier and an ion conductor. If a write voltageis reversed, the ions may suitably be dispersed away from the interface.

[0104] A programmable structure in accordance with the present inventionmay be used in many applications which would otherwise utilizetraditional technologies such as EEPROM, FLASH or DRAM. Advantagesprovided by the present invention over present memory techniquesinclude, among other things, lower production cost and the ability touse flexible fabrication techniques which are easily adaptable to avariety of applications. The programmable structures of the presentinvention are especially advantageous in applications where cost is theprimary concern, such as smart cards and electronic inventory tags.Also, an ability to form the memory directly on a plastic card is amajor advantage in these applications as this is generally not possiblewith other forms of semiconductor memories.

[0105] Further, in accordance with the programmable structures of thepresent invention, memory elements may be scaled to less than a fewsquare microns in size, the active portion of the device being less thanon micron. This provides a significant advantage over traditionalsemiconductor technologies in which each device and its associatedinterconnect can take up several tens of square microns.

[0106] Additionally, the devices of the present invention requirerelatively low energy and do not require “refreshing.” Thus, the devicesare well suitable for portable device applications.

[0107] Although the present invention is set forth herein in the contextof the appended drawing figures, it should be appreciated that theinvention is not limited to the specific form shown. For example, whilethe programmable structure is conveniently described above in connectionwith programmable memory devices, the invention is not so limited; thestructure of the present invention may suitably be employed asprogrammable active or passive devices within a microelectronic circuit.Furthermore, although only some of the devices are illustrated asincluding buffer, barrier, or transistor components, any of thesecomponents may be added to the devices of the present invention. Variousother modifications, variations, and enhancements in the design andarrangement of the method and apparatus set forth herein, may be madewithout departing from the spirit and scope of the present invention asset forth in the appended claims.

We claim:
 1. A microelectronic programmable structure comprising: an ionconductor formed of an ion conductive material and conductive ions; anoxidizable electrode proximate the ion conductor; and an indifferentelectrode proximate the ion conductor.
 2. The microelectronicprogrammable structure of claim 1, further comprising a buffer layerbetween the oxidizable electrode and the ion conductor.
 3. Themicroelectronic programmable structure of claim 2, wherein the bufferlayer comprises a material selected from the group consisting ofAg_(x)O, Ag_(x)S, Ag_(x)Se, Ag_(x)Te, where x≧2, Ag_(y)I, where y≧1,CuI₂, CuO, CuS, CuSe, CuTe, GeO₂, and SiO₂.
 4. The microelectronicprogrammable structure of claim 1, wherein the indifferent electrodecomprises platinum.
 5. The microelectronic programmable structure ofclaim 1, wherein the oxidizable electrode comprises a material selectedfrom the group consisting of a transition metal sulfide and a transitionmetal selenide.
 6. The microelectronic programmable structure of claim5, wherein the oxidizable electrode further comprises intercalatedsilver.
 7. The microelectronic programmable structure of claim 5,wherein the oxidizable electrode comprises TaS₂.
 8. The microelectronicprogrammable structure of claim 1, wherein the oxidizable electrodecomprises AgI.
 9. The microelectronic programmable structure of claim 8,wherein the oxidizable electrode comprises excess silver.
 10. Themicroelectronic programmable structure of claim 1, wherein the ionconductor comprises a solid solution selected from the group consistingof As_(x)S_(1−x)—Ag, Ge_(x)Se_(1−x)—Ag, Ge_(x)S_(1−x)—Ag,As_(x)S_(1−x)—Cu, Ge_(x)Se_(1−x)—Cu, Ge_(x)S_(1−x)—Cu, where x rangesfrom about 0.1 to about 0.5.
 11. The microelectronic programmablestructure of claim 10, wherein the ion conductor comprises a fillermaterial.
 12. The microelectronic programmable structure of claim 11,wherein the filler material comprises a dielectric and is present in theion conductor at a volume percent of up to about 50 percent.
 13. Themicroelectronic programmable structure of claim 11, wherein the fillermaterial comprises a dielectric and is present in the ion conductor at avolume percent of up to about 5 percent.
 14. The microelectronicprogrammable structure of claim 11, wherein the filler materialcomprises silver.
 15. The microelectronic programmable structure ofclaim 1, wherein the ion conductor comprises a glass having acomposition of Ge_(0.17)Se_(0.83) to Ge_(0.25)Se_(0.75).
 16. Themicroelectronic programmable structure of claim 15, wherein the ionconductor further comprises up to about 34 percent silver.
 17. Themicroelectronic programmable structure of claim 1, further comprising atransistor in contact with one of the oxidizable or the indifferentelectrodes.
 18. The microelectronic programmable structure of claim 1,further comprising a diode in contact with one of the oxidizable or theindifferent electrodes.
 19. The microelectronic programmable structureof claim 1, wherein the ion conductor is formed within a via in aninsulating material layer.
 20. The microelectronic programmablestructure of claim 19, further comprising a diode formed within the via.21. The microelectronic programmable structure of claim 19, wherein theion conductor contacts the indifferent electrode about a portion of theperimeter of the ion conductor.
 22. The microelectronic programmablestructure of claim 21, wherein the ion conductor contacts theindifferent electrode about a sloped portion of the perimeter of the ionconductor.
 23. The microelectronic programmable structure of claim 1,wherein the indifferent electrode, the oxidizable electrode, and the ionconductor are formed on a surface of an insulating material layer. 24.The microelectronic programmable structure of claim 1, wherein the ionconductor is formed within a via of a first insulating material layer,and wherein the programmable structure further comprises a secondinsulating material formed within the via.
 25. The microelectronicprogrammable structure of claim 1, wherein the ion conductor is formedalong a sidewall of a via formed within an insulating layer.
 26. Themicroelectronic programmable structure of claim 1, wherein the ionconductor is formed within a sloped via within an insulating materiallayer.
 27. The microelectronic programmable structure of claim 1,further comprising a barrier layer between the indifferent electrode andthe ion conductor.
 28. The microelectronic programmable structure ofclaim 27, wherein the barrier layer comprises a conductive material. 29.The microelectronic programmable structure of claim 27, wherein thebarrier layer comprises an insulating material.
 30. The microelectronicprogrammable structure of claim 1, wherein surface area of theindifferent electrode in contact with the ion conductor is less than thesurface area of the oxidizable electrode in contact with the ionconductor.
 31. The microelectronic programmable structure of claim 1,wherein an interface between the indifferent electrode and the ionconductor is roughened.
 32. A multi-cell programmable microelectronicdevice comprising: a first electrode of a first type; a second electrodeof a second type; a first ion conductive material of a first resistanceinterposed between the first electrode and the second electrode; a thirdelectrode of a first type; and a second ion conductive material of asecond resistance interposed between the second electrode and the thirdelectrode.
 33. The multi-cell programmable microelectronic device ofclaim 32, wherein the first and third electrodes comprise an indifferentelectrode material and the second electrode comprises an oxidizableelectrode material.
 34. The multi-cell programmable microelectronicdevice of claim 32, wherein the first and third electrodes comprise anoxidizable electrode material and the second electrode comprises anindifferent electrode material.
 35. A multi-cell programmablemicroelectronic device comprising: a plurality of electrodes of a firsttype; a plurality of electrodes of a second type; and a plurality of ionconductor structures, wherein at least one of the plurality of ionconductor structures is interposed between one of the plurality ofelectrodes of a first type and one of the plurality of electrodes of asecond type, and wherein a plurality of electrodes of a first type areelectrically coupled together.
 36. The multi-cell programmablemicroelectronic device of claim 35, wherein the plurality of electrodesof a first type comprise oxidizable electrode material.
 37. Themulti-cell programmable microelectronic device of claim 35, wherein theplurality of electrodes of a first type comprise indifferent electrodematerial.
 38. The multi-cell programmable microelectronic device ofclaim 35, wherein at least a portion of the plurality of ion conductorstructures are formed within a via within an insulating material layer.39. The multi-cell programmable microelectronic device of claim 35,wherein at least a portion of the plurality of ion conductor structuresare formed on a surface of an insulating material layer.
 40. A method offorming a programmable microelectronic structure, the method comprisingthe steps of: providing a substrate; forming a layer of electrodematerial of a first type overlying the substrate; forming an insulatinglayer overlying the layer of electrode material of a first type; forminga via through the insulating layer and the layer of electrode materialof a first type; depositing ion conductor material into the via; andforming an electrode of a second type overlying the ion conductormaterial.
 41. The method of claim 40, wherein the step of forming a viaincludes isotropically etching the insulating layer.
 42. The method ofclaim 40, wherein the step of forming a via includes anisotropicallyetching the insulating layer.
 43. The method of claim 40, wherein thestep of forming a via includes isotropically etching the layer ofelectrode material of a first type.
 44. The method of claim 40, whereinthe step of forming a via includes anisotropically etching the layer ofelectrode material of a first type.
 45. The method of claim 40, furthercomprising the step of applying a bias across the electrode material ofthe first type and the electrode material of the second type tomanipulate a concentration of conductive material in the ion conductor.46. The method of claim 40, further comprising the step of applying abias across the electrode material of the first type and the electrodematerial of the second type to manipulate an amount of conductivematerial present in one of the electrode material of the first type andthe electrode material of the second type.
 47. The method of claim 40,wherein the step of depositing ion conductor material comprisesdepositing germanium onto a surface and reacting the germanium withH₂Se.
 48. The method of claim 40, wherein the step of depositing ionconductor material comprises depositing arsenic onto a surface andreacting the arsenic with H₂Se.
 49. The method of claim 40, wherein thestep of depositing ion conductor material comprises depositing germaniumonto a surface and reacting the germanium with H₂S.
 50. The method ofclaim 40, wherein the step of depositing ion conductor materialcomprises depositing arsenic onto a surface and reacting the arsenicwith H₂S.
 51. A method of forming a programmable microelectronic device,the method comprising the steps of: forming an ion conductor structureoverlying a substrate; depositing an electrode material layer overlyingthe ion conductor structure; and patterning the electrode material layerto form electrodes in contact with the ion conductor structure.
 52. Themethod of claim 51, wherein the step of forming an ion conductorstructure comprises depositing germanium onto a surface and reacting thegermanium with H₂Se.
 53. A method of forming an electronic device, themethod comprising the steps of: forming a first electrode on a surfaceof a substrate; depositing a first insulating layer over a surface of athe first electrode; forming a via in the first insulating layer;depositing a second insulating material within a portion of the via;depositing ion conductor material within a portion of the via; andforming a second electrode overlying the ion conductor.
 54. The methodof forming an electronic device of claim 53, wherein the step ofdepositing ion conductor material comprises the step of deposing the ionconductor material within a via formed in the second insulatingmaterial.
 55. The method of forming an electronic device of claim 53,wherein the step of depositing a second insulating material comprisesusing a directional deposition technique.
 56. The method of forming anelectronic device of claim 53, wherein the step of depositing an ionconductor material comprises forming a conformal layer of ion conductormaterial.
 57. The method of forming an electronic device of claim 53,further comprising the step of removing a portion of the ion conductormaterial from a surface of the first insulating material.
 58. A methodof forming a multi-cell programmable device, the method comprising thesteps of: forming a first electrode on a surface of a substrate; forminga first ion conductor portion overlying the first electrode; forming asecond electrode overlying the first ion conductor portion; forming asecond ion conductor portion overlying the second electrode; and forminga third electrode overlying the second ion conductor portion.
 59. Amethod of forming a glass composition, the method comprising the stepsof: selecting an ampoule; cleaning the ampoule using hydrofluoric acid;drying the ampoule for about 24 to about 120 hours at about 120° C.;evacuating the ampoule; heating the ampoule until the ampoule turns red;filling the ampoule with a charge; heating the ampoule to a temperaturebelow the melting temperature of the glass constituents; ramping thetemperature at a rate of about 0.5 degrees per minute to a temperatureabout 50° C. higher than the liquidus temperature of the glass; and slowrocking the glass composition at a rate of about 20 per minute for aperiod of about six hours.